1. Field of the Invention
This invention is related in general to the field of optical screens. In particular, it relates to scintillation screens converting high-energy electrons to photons used in transmission electron microscopy.
2. Description of the Related Art
Modern digital imaging systems are built around well-known charged-coupled-device (CCD) detectors possessing high sensitivity, wide dynamic range and speed, and providing direct access to output digital data. Certain applications, however, cannot use CCDs for direct detection of the flux of particles that form the image. For example, direct CCD registration of high-energy electrons used in transmission electron microscopy (TEM) is not practical due to saturation and even radiation damage to the CCDs induced by the electron flux. This is why conventional CCD-based systems for detection of high-energy particles primarily rely on energy conversion that precedes the detection step. The conversion is typically achieved with the use of a so-called scintillator, which is a layer of appropriate material—such as one of the specific phosphors used in the art—that is directly illuminated by the high-energy primary electrons forming the image in the TEM and that generates photons in response to such irradiation. The photons are later relayed to the CCD camera using conventional means, the most common being optical-lens or fiber-optic systems, as seen in FIGS. 1A and 1B. In a typical lens-based system, the scintillating phosphor layer is usually formed and contained separately from the optics, as shown in FIG. 1A, while in fiber-optic (FO) based systems it is usually deposited directly on an appropriately prepared input surface of the FO bundle, as seen in FIG. 1B.
In either conventional implementation, however, a serious practical problem arises from the use of a scintillating (phosphor) layer. When penetrating through the phosphor, the fast electrons are being randomly scattered as they generate light and deviate from their original paths. As a result, the spatial extent of photon distribution generated by any electron in the scintillator is significantly larger than the original electron distribution (which is basically a delta-function), leading to a worsening of image resolution during the step of electron-to-photon conversion. This process is illustrated in the FO-based system of FIG. 2, where multiple channels feed each pixel. As shown in the figure, the light distribution L significantly exceeds the extent of a single FO channel, as each electron e− impinging on the scintillating layer S delivers corresponding photons to the CCD detector by not one but several FO channels. As a result, several CCD pixels are illuminated by the image of the single electron due to random scattering of the electron (as well as generated X-rays and photons) in the scintillating layer S and overall image resolution is correspondingly decreased. It is clear, therefore, that only when the light distribution L and the CCD pixel have comparable sizes the digital imaging provided by the CCD is optimized.
Therefore, there exists an unresolved need for a system of electron-to-photon energy conversion in a scintillator that does not decrease the image resolution beyond the limit of the subsequent optical train delivering the light to the CCD detector. This invention solves this problem by using a discretized scintillator screen, arranged in pixel-like fashion, where the “pixels” of the screen are cells filled with scintillating phosphor and separated by barriers impenetrable to both primary electrons and the light generated inside the phosphor. As a result, the distribution of the light generated within a given screen cell is controlled by the geometric extent of the cell, and loss of imaging resolution due to electron-to-photon conversion in the scintillator layer is thereby minimized. It is preferred that each screen cell be smaller than a CCD pixel so that the CCD pixel defines the spatial resolution of the imaging system.